The conversations focused on Paul Manafort, the Trump campaign chairman at the time, and Michael T. Flynn, a retired general who was advising Trump, the officials said. The letter also narrows the scope of the documents the panel is seeking.
Burr said if there is no response from Flynn his committee may consider a contempt-of-Congress charge.
"Everything is on the table", Burr said. Flynn had rejected the earlier subpoena for records as being so broad that providing them could make him vulnerable.
The leaders of the US Senate Intelligence Committee said they will subpoena two of Flynn's businesses after he declined to comply with a subpoena for documents in the panel's Russian Federation probe. According to the semi-official narrative, the Federal Bureau of Investigation learned of hacking attacks on the Democratic National Committee in the summer of 2015 and the attacks were supposedly linked soon after to two units of Russian military intelligence.
Brennan also said that the Russians had contacts with the staff of the trump in 2016, according to CNN. Flynn has refused to hand over that information. He says the committee will be "following up with subpoenas" to maximize the chances of getting information from Flynn.
In another development, President Trump has retained Marc Kasowitz, a New York-based trial lawyer who has represented President Trump in the past, as his private attorney for special counsel Robert Mueller's investigation into the Russian Federation matter, a Fox Business Network reporter said on Twitter.
However, Brennan said in testimony before the House Intelligence Committee that he did not know if the interactions amounted to collusion. Prior to that, Brennan was a top counterterrorism and homeland security adviser to President Barack Obama.More news: Those responsible for leaks of Manchester attack deatils will face prosecution
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A new report says USA intelligence services obtained information during the 2016 presidential election that showed senior Russian officials discussing how to influence Donald Trump through his campaign advisers. Other clues included direct conversations between members of the Trump campaign and Russian officials.
Brennan said that he can not say definitively there was actual collusion. Brennan said the use of spear phishing, and "whatever else so that they can then gain access to people's emails, computer systems networks", is something that the Russians are adept at. The website released material hacked from email accounts of the Democratic National Committee and Hillary Clinton campaign officials during last year's presidential campaign. And he said there appears to be "very, very damaging leaks, and I find them appalling and they need to be tracked down".
Brennan says that Russian protests that they are not working with WikiLeaks and WikiLeaks protests that they are not working with the Russians are both "disingenuous".
Schiff, a former prosecutor, said the Fifth Amendment does not protect Flynn from certain document and material requests and his committee will obtain what it needs 'from Mr. Flynn or his businesses in whatever means we need to'.
Brennan also meanwhile addressed news reports that Trump, in an Oval Office meeting this month with the Russian foreign minister and ambassador, shared highly classified information provided by a USA ally about the Islamic State group.
A group of Democrats on the House Financial Services Committee, said on Wednesday it had asked Deutsche Bank to provide information on whether any accounts connected to Trump had ties to Russian Federation.
"The goal was to ensure that experts in key agencies had access to information and intelligence relevant to Russian actions so that we could have as full an appreciation as possible on the scope, nature, and intentions of this Russian activity", Brennan said.
A group of Democrats on the Senate Banking Committee has voted against confirming a high-ranking Treasury Department official.